The present invention relates to a superresolving optical apparatus, applicable to optical disc systems, and the like. More particularly, the invention is concerned with an optical apparatus which has a high optical utilization ratio and is yet capable of electrically altering a numerical aperture thereof with ease, with respect to optical discs whose proper numerical apertures for image formation differ from each other, such as DVDs (digital versatile discs), CDs (compact discs), and the like.
A theoretical resolution limit and a numerical aperture of an optical system is briefly described hereinafter to facilitate an understanding of the conventional technologies concerned.
In an optical system designed to have little aberration according to geometrical optics, a focused point must in theory be infinitely small in size. However, it has, in fact, a spatial spread in a finite size due to the effect of optical diffraction owing to the wave motion characteristic of light.
Provided that a numerical aperture of an optical system, contributing to optical image formation or condensing of light, is designated NA, the spatial spread of a focused point is defined by the following formula:
kxc3x97xcex÷NAxe2x80x83xe2x80x83(1)
where xcex is the wavelength of light, and k is a constant for respective optical systems (a value, normally in the range from 1 to around 2). Further, the numerical aperture NA is proportional to a ratio of the diameter D of an effective entrance pupil of an optical system (generally the diameter of an effective light beam) to a focal length f, that is: D/f.
The spatial spread of the focused point as expressed by formula represents a theoretical resolution limit of the optical system, and is also called a diffraction limit.
As is evident from formula, a theoretical resolution may be enhanced by the use of a light beam at a shorter wavelength xcex, or by enlarging the numerical aperture NA of an optical system. However, a short wavelength light source is generally complex in construction, and higher in production cost.
This tendency becomes more pronounced, particularly, in the case of a laser light source used for optical disc systems, photolithographic masking systems, and the like. Further, the greater the numerical aperture NA of an optical system is, the more the optical system becomes prone to aberration according to geometrical optics. Accordingly, for recording information on common optical disc systems, a semiconductor laser for emitting a light beam at a wavelength in the order of 700 nm is used as a light source while a condensing optics having the numerical aperture NA on the order of 0.5 is used.
As the conventional technology capable of achieving superresolution by the use of a light source and condensing optics as described above, a superresolving optical system constructed such that a portion of an effective light beam falling on the condensing optics is shielded with a shielding band (shielding plate) is well known (reference: Japanese Journal of Applied Physics, Vol. No. 28 (1989) Supplement 28-3, pp. 197-200). With this superresolving optical system using the shielding plate, a focused spot size is rendered narrower by 10 to 20% with respect to the theoretical resolution limit of the optical system.
However, shielding a portion of the effective light beam falling on the condensing optics by means of the shielding plate will result in a lower optical utilization ratio. Furthermore, with the superresolving optical system described above wherein the central region of a light beam, including the optical axis, is shielded with the shielding plate, degradation in the optical utilization ratio becomes further pronounced because the central region of the light beam generally belongs to a higher intensity zone according to the distribution of light intensity.
Such a low optical utilization ratio inevitably requires the use of a light source capable of outputting higher power, resulting in a higher cost of an optical apparatus because such a high power output light source is expensive. Particularly, for application to optical disc systems, a semiconductor laser light source, expensive even at a low power output, is used, and consequently, it is practically impossible to employ a high power output light source from a cost point of view.
Further, when a portion of the effective light beam falling on the condensing optics is shielded by means of the shielding plate, sidelobes typical of a superresolution phenomenon occur on both sides of the focused spot. According to the technology referred to in the literature described in the foregoing, the sidelobes are shielded by installing a slit at a position where signal light reflected from the focused spot is condensed by a condenser lens, so that a focused spot with the sidelobes substantially removed is formed by another condenser lens installed after the slit.
However, for condensing light by an additional condenser lens, an additional optical path of the optical system is required to that extent, and the number of components of the optical system increases, thereby causing a configuration of the optical system to become more complex. Further, delicate positioning of the slit is required because any deviation in the position of the slit will result in shielding of not only the sidelobes but also the focused spot. In addition, there will arise a problem of dust and the like sticking to a gap in the slit.
Then, even if the slit is installed in a given position, the fact remains that light is shielded by the slit, and consequently, diffraction of light occurs again, resulting in the occurrence of some sidelobes. Further, shielding of light by means of the shielding plate naturally leads to a significant degradation in the optical utilization ratio.
The invention has been developed in light of the circumstances described above, and the first object of the invention is to achieve detection of the signal light reflected from the focused spot of superresolution without causing degradation in the optical utilization ratio while solving a problem of sidelobe as well.
Further, as the formula (1) shown in the foregoing clearly indicates, a theoretical resolution of an optical system is largely dependent on a numerical aperture thereof. A numerical aperture of a condenser (objective) lens of an optical pickup used in optical disc systems is usually in the order of 0.45 in the case of CDs and CD-ROMs, and in the order of 0.55 in the case of DVDs (digital versatile discs). Meanwhile, an optical disc substrate has a thickness of about 1.2 mm for use in CDs, and about 0.6 mm for use in DVDs. The condenser lens of an optical pickup that is required of condensing light up to the diffraction limit is designed by taking into account even a thickness of the optical disc substrate. Hence, a proper numerical aperture of the condenser lens for use in CDs or CD-ROMs is different from that of the condenser lens for use in DVDs, thus preventing common use of the optical pickup therebetween.
Accordingly, in order to overcome this problem, there have been in use various conventional methods such as a method of installing two units of optical pickups in one optical apparatus, a method of creating two focal points by making a condenser lens of an optical pickup to be imprinted with a hologram, a method of switching over the diameter of an effective entrance pupil by use of a liquid crystal shutter, and so forth.
However, if two units of optical pickups are installed in one optical apparatus, this will result in a complex configuration of the optical apparatus, leading to an increase in manufacturing cost. If two focal points are created by imprinting the condenser lens with a hologram, it follows that one unnecessary focused spot at either of the focal points will always occur, resulting in a degradation of the optical utilization ratio. This will pose a problem with devices requiring a large light amount such as DVD-RAMs, that is, writable and rewritable DVDs. Similarly, the method of using the liquid crystal shutter also will pose the same problem of degradation in the optical utilization ratio, because a portion of transmitted light is absorbed by polarizers making up the liquid crystal shutter.
The second object of the invention is to enable the use of a common optical apparatus (optical pickup) with respect to optical discs having differing proper numerical apertures for image formation such as DVDs, CDs, and so forth, and to enable numerical apertures to be electrically switched with ease without causing much degradation in the optical utilization ratio.
The present invention has been developed in order to attain the first and second objects described above by using substantially common means.
That is, with an optical apparatus according to the invention, comprising a condensing optics for condensing incident light of linearly polarized light, an optical splitting element for splitting reflected light reflected by a light reflection member disposed in close proximity of the focal plane of the condensing optics from the incident light, and an optical detection element for detecting a split light beam split by the optical splitting element, in order to attain the first object described above, an optical rotatory element capable of optically rotating substantially 90xc2x0 a polarization axis of the linearly polarized light in a region corresponding to a portion of an effective light beam of the linearly polarized light, available for the condensing optics, and also controlling an optical rotatory power thereof by electric signals is disposed in the above-described optical path of the incident light of the linearly polarized light, and a linearly polarized light detection element is disposed in the optical path of the above-described split light beam.
Thus, by disposing the optical rotatory element capable of controlling the optical rotatory power thereof by use of electric signals in the optical path of the incident light of the linearly polarized light, and by dividing the effective light beam of the linearly polarized light into the regions where the polarization axes thereof cross each other at right angles, a superresolving optical system is established without shading, in theory, the light beam. Further, by disposing the linearly polarized light detection element in the optical path for the reflected light formed after a superresolved focused spot is reflected by the light reflection member, sidelobes can be removed from reflected signal light without the use of a slit and a condenser lens.
It is desirable that the direction of the detection axis of the linearly polarized light detection element is set in the range of minus 85xc2x0 to minus 5xc2x0 or from 5xc2x0 to 85xc2x0, that is, in a range excluding around minus 90xc2x0, 0xc2x0, and 90xc2x0, with respect to the direction of the polarization axis of the linearly polarized light falling on the optical rotatory element.
It is preferable that a liquid crystal element wherein a region having a function of optically rotating the polarization axis of the linearly polarized light 90xc2x0 and a region not having such an optically rotatory function are caused to occur by applying a voltage to liquid crystals in a portion of the light transmitting region thereof is used for the optical rotatory element, and the liquid crystal element is disposed such that an alignment direction of liquid crystal molecules on a side of the liquid crystal element, where the linearly polarized light falls, is set to coincide with, or cross at right angles to the direction of the polarization axis of the linearly polarized light.
As the liquid crystal element, a 90xc2x0 twisted-nematic liquid crystal element having transparent electrodes for applying a voltage to liquid crystals in a portion of the light transmitting region can be used, so that liquid crystal molecules in the portion to which a voltage is applied via the transparent electrodes turn into homeotropic alignment, thereby losing the 90xc2x0 optical rotatory power thereof.
A portion of a region where the polarization axis of the linearly polarized light is optically rotated 90xc2x0 by the optical rotatory element, preferably corresponds to a substantially circular region centering around the optical axis of the linearly polarized light, or a region other than the substantially circular region, within the effective light beam available for the condensing optics.
Otherwise, a portion of the region where the polarization axis of the linearly polarized light is optically rotated 90xc2x0 by the optical rotatory element may correspond to a substantially oblong region centering around the optical axis of the linearly polarized light, or a region other than the substantially oblong region, within the effective light beam available for the condensing optics.
In the cases described above, an area of the substantially circular region, occupied within the effective light beam, is preferably in the range of 1 to 20% of an area on a plane orthogonal to the optical axis, occupied by the effective light beam.
Further, in the case of an oblong region, an area of the substantially oblong region, occupied within the effective light beam, is preferably in the range of 10 to 40% (more preferably about 20%) of an area on a plane orthogonal to the optical axis, occupied by the effective light beam.
With the optical apparatus according to the invention, wherein the region where the polarization axis of the linearly polarized light is optically rotated 90xc2x0 by the optical rotatory element corresponds to the substantially circular region centering around the optical axis of the linearly polarized light, or the region other than the substantially circular region, within the effective light beam available for the condensing optics, the second object of the invention described above can be attained by disposing the linearly polarized light detection element having the detection axis thereof, which is oriented so as to substantially coincide with or to cross at right angles the direction of the polarization axis of the incident linearly polarized light, in the optical path of the split light beam, split from the incident light by the optical splitting element after a superresolved focused spot is reflected by the light reflection member.
That is, the optical apparatus of the invention can be used as an optical pickup for common use in DVDs and CDs etc, for which different proper numerical apertures are required for image formation, and when the linearly polarized light passing through the substantially circular region or the region other than the substantially circular region is optically rotated 90xc2x0 by controlling the optical rotatory power of the optical rotatory element by use of electric signals, the substantial numerical apertures can be switched over without causing degradation in the optical utilization ratio. Furthermore, sidelobes can be removed from the reflected signal light by the agency of the linearly polarized light detection element.
In this case, an area of the substantially circular region, occupied within the effective light beam, is preferably set to be in the range of 50 to 80% (more preferably about 70%) of an area on a plane orthogonal to the optical axis, occupied by the effective light beam.